Probably doing nothing wrong - manually focusing a modern camera designed to be used with AF is not easy.
They have smaller viewfinders (because of the crop factor) and are often darker. The focusing screens are not optimised for low f/numbers, and though you might be able to change this for a better one, it won't be quite so bright for normal shooting. And then, depending on the lens, the manual focusing mech may not fine enough for really critical accuracy.
Your best bet is probably a different focusing screen, plus practise.
Having said that, manually focusing an old full frame film SLR was never that easy.